1. Field of the Invention
The present invention relates to a vacuum hold-down device for holding a workpiece or component for machining.
2. Description of the Prior Art
Vacuum devices for holding work objects have been known for some time. DE 4221222 describes a vacuum hold-down device comprising a base plate with suction openings connected to a vacuum system. A top plate on which work objects are laid has vertical holes from the underside right through to the upper surface and there is a seal surrounding these holes under the underside and another similar seal on the top surface. The holes in the base plate not overlain by the top plate are closed with stoppers, and the base plate is provided with threaded bushes for fixing clips. The device is complex to construct, and few small workshops have suitable vacuum lines for connection to the suction openings.
Also known are systems which employ pistons to create a partial vacuum to hold a workpiece, as described in U.S. Pat. No. 5,626,378, U.S. Pat. No. 4,470,585, GB 1,588,012, GB 2 120 141, DE 42 15 140, and SU 1079373. The pistons act on a deformable material, for example a flexible membrane or a suction cup, which in turn subjects the workpiece to a partial vacuum. In another application, FR 2 262 751 describes the use of a piston to clamp a workpiece to marble. The use of pistons, and means to return the pistons to a rest position adds to cost and manufacturing complexity. Moreover, flexible suction cups or membranes do not work well with steel workpieces; swarf associated with the machining process rapidly causes damage to the flexible material of the membrane or suction cup.
FR 2 546 790 discloses a vacuum hold-down device which comprises a sealed box which can be evacuated by a venturi and which has holes in its top which are fitted with retractable seals for forming a seal between the upper surface and an object to be held. The box is obtained either my manufacturing by removing shavings or by moulding. The waterproof assembly of its elements or parts is effected by using thixotropic products on the sealing parts. The device requires significant downtime when parts which have been machined must be removed and replaced.
The present invention seeks to reduce at least some of the problems of the prior art devices.